We can do a rough estimate based on a GTX 960 in my cluster. I have an NVIDIA GeForce GTX 960 (2048MB) which contributes 120,000 RAC (Recent Average Credit) of the total 775,000 RAC I have in 'Moo! Wrapper'. It is running with stock settings.

The total income I earn from this project is 38 GRC/day. That means the GTX960 is making about 6 GRC/day, or 180 GRC/month. I have only been running this card for about 2 weeks though, and RAC does not cap until about a month, so the GTX960 making 200GRC/month in Moo! Wrapper seems like a fair estimate.

You could probably make a bit more if you ran Primegrid or Amicable Numbers. You could also earn more if you set up your GPU to run OpenCL instead of CUDA jobs (CUDA is NVIDIA exclusive and less efficient in Moo! Wrapper). The latter is for advanced users though.